The present invention relates to a process for coating a metal substrate with a surface coating applied by a fluidized-bed process after the said substrate has been coated with a primer powder.
The fluidized-bed immersion process requires a preliminary preheating of the metal article to be coated before it is immersed in the porous-bottomed vessel in which the coating powder in suspension is kept in suspension by circulating air. The powder then melts in contact with the hot metal surfaces and forms a deposit whose thickness is a function of the temperature of the substrate and of its period of immersion in the powder.
The preheating of the metal article usually takes place in an oven at a temperature which is determined primarily according to the nature and the thickness of the article to be coated.
However, for a given article, there is a minimum temperature below which it is not possible to obtain a coating of good quality from the viewpoint of its appearance and of its adhesion to the substrate.
However, on the other hand, an excessively high preheating temperature can be detrimental, more particularly in the case where the metal article has been coated beforehand with an adhesion primer which can be adversely affected by the temperature when the article passes through the oven and can no longer ensure the adhesion to the surface coating and to the metal substrate.
At the present time, the adhesion primers for surface coatings which are applied by immersion in a fluidized bed of surface coating powders exist exclusively in liquid form. That is to say, the primer resins are either in suspension or in solution in one or more solvents. The coating of metal substrates with these liquid primers is done, for example, with the aid of a pneumatic gun.
These solvents, frequently toxic to the environment, make it necessary to provide recovery and/or recycling systems thus entailing an additional plant cost.